Method and apparatus for extending the operating range of a flyforward converter

ABSTRACT

A technique for extending the operating range of a flyforward converter to low input voltages. In one aspect, power converter includes a positive input supply rail and a negative input supply rail. A power converter input voltage is to be applied between the positive and negative input supply rails. A flyback energy transfer element having a flyback input winding and a forward energy transfer element having a forward input winding are also included. The flyback and forward input windings are coupled between the positive and negative input supply rails. Voltage control circuitry coupled to the forward energy transfer element is also included to reduce a voltage across the forward input winding, substantially to zero, when the power converter input voltage falls below a first threshold value.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to power converters and, morespecifically, the present invention relates to flyforward powerconverters.

2. Background Information

Power conversion circuits are typically designed to meet cost andefficiency targets defined at the start of a design. The type of powerconversion circuit adopted in a particular design determines the overallsystem cost and operating performance.

One circuit configuration that provides the advantages of highefficiency and low system cost is a power conversion circuit called aflyforward converter. This circuit configuration effectively combineselements of two commonly used converter configurations, the flybackconverter and the forward converter.

The flyforward converter includes individual flyback and forwardtransformers or energy transfer elements each having an input windingand at least one output winding. One of the advantages of the flyforwardconverter is circuit simplicity since only one power switch is required,which is coupled to the flyback and forward energy transfer elementinput windings across an input supply rail to the power conversioncircuit. This power switch, switches on and off at a frequencydetermined by a control circuit coupled to the power switch. Thefrequency, which is the reciprocal of a switching cycle period, at whichthe power switch switches on and off may be fixed or variable dependingon the type of control circuitry adopted.

The flyforward converter provides the combined advantages of efficientuse of the flyback and forward energy transfer elements, low RMS currentin the power switch and low ripple current in capacitors, which arecoupled across the outputs of the flyback and forward energy transferelements, as will be known to one skilled in the art.

However, the flyforward configuration suffers from a limitation in itsoperating characteristic, which restricts its use in many practicalcircuits. To describe this limitation, it is convenient to regard theflyforward converter in terms of the flyback energy transfer element andforward energy transfer element individually. In order for the forwardenergy transfer element to deliver energy to the power conversioncircuit output, it is important that the magnetic flux in the forwardenergy transfer element at the end of a switching cycle period is resetto substantially the same value as it had at the beginning of theswitching cycle period before the power switch is switched on.

During the following description the flux in the magnetic core of theforward energy transfer element at the beginning of a switching cycleperiod may be referred to as the initial value of the flux. In meetingthis criterion, the magnetic core of the forward energy transfer elementis prevented from saturating. In order for this operating criterion tobe met, a reset voltage appears across the forward energy element inputwinding during the period of each switching cycle that the power switchis off.

To prevent the forward energy transfer element from saturating, theintegral of this reset voltage during the power switch off time periodis equal to the magnitude of the integral of the voltage appearingacross the forward energy transfer element input winding during thepower switch on period. This requirement is often referred to as thevolt-second balance and ensures the magnetic flux does not build up inthe magnetic core over a number of switching cycle periods, which wouldresult in saturation of the magnetic core.

During the normal operation of a forward converter, in order to maintainthe regulation of the voltage across the power conversion circuitoutput, the power switch on period as a percentage of the overallswitching cycle period, which is referred to as the duty cycle,increases as the input voltage to the power converter decreases. Therequirement to maintain the volt-second balance therefore requires thatthe magnitude of the reset voltage, integrated over the power switch offtime, increases as the power converter input voltage decreases. Thisincreased reset voltage increases voltage stress on the power switch aswell as voltage stress on rectification diodes coupled to the outputwinding of the forward energy transfer element.

This typically limits the use of the flyforward converter toapplications where the range of input voltage applied to the input ofthe power conversion circuit is very limited. This is a severelimitation since many applications having limited input voltage rangespecifications under normal operating conditions, have short termtransient operating conditions where low input voltage must be toleratedwith the power converter remaining fully operational.

Examples of applications where this could be a requirement aretelevision and personal computer power conversion circuits. In theseapplications, if the input voltage to the power conversion circuit fallsbelow the normal operating value, the power conversion circuit continuesto operate long enough that memory back-up and other housekeepingfunctions can be completed by electronic circuitry coupled to the outputof the power converter, before the power conversion circuit outputvoltage becomes too low. The period of time for which the powerconversion circuit operates under these conditions is often referred toas the hold-up period. Although this is only a transient condition inthe operation of the power converter, the limitations of the flyforwardconverter discussed above, make it necessary to rate the voltage of thepower switch and output rectifiers to withstand this transientcondition. This limitation can greatly increase the cost of the overallconverter, making other converter topologies more attractive for thisreason alone.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention detailed illustrated by way of example and notlimitation in the accompanying figures.

FIG. 1 is a schematic illustrating a flyforward converter

FIG. 2 is a schematic of one embodiment of a circuit in accordance withthe teachings of the present invention.

FIG. 3 is a schematic of another embodiment of a circuit in accordancewith the teachings of the present invention.

FIG. 4 is a schematic of yet another embodiment of a circuit inaccordance with the teachings of the present invention.

FIG. 5 is a schematic of still another embodiment of a circuit inaccordance with the teachings of the present invention.

FIG. 6 is a schematic of still another embodiment of a circuit inaccordance with the teachings of the present invention.

FIG. 7 is a schematic of still another embodiment of a circuit inaccordance with the teachings of the present invention.

DETAILED DESCRIPTION

A novel technique to extend the operating range of a flyforwardconverter is disclosed. In the following description, numerous specificdetails are set forth in order to provide a thorough understanding ofthe present invention. It will be apparent, however, to one havingordinary skill in the art that the specific detail need not be employedto practice the present invention. In other instances, well-knownmaterials or methods have not been described in detail in order to avoidobscuring the present invention.

In general, a simple and novel technique for extending the operatingrange of a flyforward converter is provided according to embodiments ofthe present invention. In various embodiments, either the voltage acrossthe forward energy transfer element input winding is reducedsubstantially to zero or the voltage across the flyback energy transferelement input winding is increased to substantially equal the powerconverter input voltage during the on period of a power switch, when thepower supply input voltage falls below a threshold value in accordancewith the teachings of the present invention.

In one embodiment this is achieved by coupling a switch across theforward energy transfer element input winding, which is turned on whenthe power converter input voltage falls below a threshold value. Inanother embodiment this is achieved by coupling a switch across anoutput winding of the forward energy transfer element, which is turnedon when the power converter input voltage falls below a threshold value.In one embodiment this is achieved by causing the forward energytransfer element to saturate when the power converter input voltagefalls below a threshold value.

To illustrate, FIG. 1 shows a schematic of a flyforward power converter100. Flyforward power converter input voltage Vin 102 is applied betweenpositive input supply rail 101 and negative input supply rail 103.Flyback energy transfer element 112 and forward energy transfer element110 are coupled between input rails 101 and 103 as shown. Flyback energytransfer element 112 has a flyback input winding 105 and forward energytransfer element 110 has a forward input winding 106. Power switch 108is shown coupled to forward input winding 106.

The order in which the energy transfer elements are coupled betweeninput rails 101 and 103 and which of the input windings 105 or 106 powerswitch 108 is coupled to, is not important to the operation orunderstanding of the flyforward converter configuration or the teachingsof the present invention.

Power switch 108 is shown directly coupled to the negative supply rail103 in FIG. 1. However, the converter operation would be unaffected ifpower switch 108 were instead coupled directly to positive supply rail101 or between input windings 105 and 106. Control circuit 107 iscoupled to power switch 108 and to feedback circuit 118, which is inturn coupled to output rails 115 and 117 and typically detects voltagebetween the power converter output rails though it can also detectoutput current flowing at the power converter circuit output or otherparameters depending on the converter purpose and design.

Control circuit 107 and power switch 108 may be separate discretecircuit elements or form part of an integrated circuit as illustrated byborder 109. Control circuit 107 and power switch 108 may bemonolithically integrated within integrated circuit 109. An integratedcircuit including circuit elements 107 and 108 can additionally includeother functional blocks not shown here so as not to obscure theteachings of the present invention. These considerations of integrationapply to all converter embodiments discussed below.

Control circuit 107 is responsive to a signal provided by feedbackcircuit 118 and determines the period of time for which the power switch108 is turned on and off during a switching cycle period to regulate anoutput of the power converter. The percentage of time within a switchingcycle period for which power switch 108 is on, is called the duty cycle.

Clamp/reset circuit 104 is shown coupled to forward input winding 106and power switch 108 and input supply rails 101 and 103. In practice,depending on the type of clamp/reset circuit being used, the clamp/resetcircuit 104 may be coupled to only one of the input supply rails 101 or103. The clamp/reset circuit 104 performs the dual functions of limitingthe maximum voltage applied across power switch 108 when it is off andensuring the magnetic flux in the forward energy transfer element 110 isreset to its initial value before the beginning of the next switchingperiod when control circuit 107 turns on power switch 108.

From FIG. 1, if the voltage across power switch 108 is assumed to bezero during the period that it is on:V _(IN) =V _(FLY) +V _(FWD)  (1)where V_(FLY) 121 and V_(FWD) 122 are the applied voltages across theinput windings of energy transfer elements 112 and 110 respectively. Ifthe power switch 108 on and off times are defined as T_(ON) and T_(OFF),respectively, the peak voltage across the forward input winding 106during the off time of power switch 108, to ensure that the magneticflux in forward energy transfer element 110, is reset substantially toits initial value before the beginning of the next switching periodfollows the requirement: $\begin{matrix}{V_{RESET} \geq \frac{V_{FWD} \times T_{ON}}{T_{OFF}}} & (2)\end{matrix}$This is the minimum peak value of the reset voltage. If the resetvoltage were constant throughout the off period of the power switch,then (2) would define the minimum value of this constant reset voltageto reset the magnetic core of the forward energy transfer element.V_(RESET) may be a fixed reference value to which the voltage across theinput winding of the forward input winding is clamped during the offtime if power switch 108, in which case its value also obeys therelationship in (2).

The duty cycle D is defined by the relationship: $\begin{matrix}{D = \frac{T_{ON}}{T_{ON} + T_{OFF}}} & (3)\end{matrix}$From equation (3), substituting for T_(ON)/T_(OFF) in equation (2)yields the following requirement: $\begin{matrix}{V_{RESET} \geq \frac{V_{FWD}}{\left( {1 - D} \right)}} & (4)\end{matrix}$

In a flyforward converter, the input to output turns ratio of theflyback energy transfer element is $\begin{matrix}{n_{FLY} = \frac{N_{PFLY}}{N_{SFLY}}} & (5)\end{matrix}$where N_(PFLY) and N_(SFLY) are the number of input and output turnsrespectively of the flyback energy transfer element.

The input to output turns ratio of the forward energy transfer elementis $\begin{matrix}{n_{FWD} = \frac{N_{PFWD}}{N_{SFWD}}} & (6)\end{matrix}$where N_(PFWD) and N_(SFWD) are the number of input and output turnsrespectively of the forward energy transfer element.

As will be known to one skilled in the art, in continuous conductionmode, the voltage conversion ratio of the fly-forward converter is:$\begin{matrix}{\frac{V_{OUT}}{V_{IN}} = \frac{D}{{n_{FLY}\left( {1 - D} \right)} + {n_{FWD}D}}} & (7)\end{matrix}$Equation (7) can be rearranged, making the duty cycle D the subject:$\begin{matrix}{D = \frac{n_{FLY}}{n_{FLY} - n_{FWD} + \frac{V_{IN}}{V_{OUT}}}} & (8)\end{matrix}$

As can be observed from equation (8) for a given converter design, sinceall other elements are constant, when VIN falls, the duty cycle Dincreases.

Since, from equation (4), the reset voltage increases as D increases, toensure the forward energy transfer element is reset before the nextpower switch 108 on time, it follows that the reset voltage V_(RESET)increases as V_(IN) is reduced.

The clamp/reset circuit 104 of FIG. 1 is therefore designed to sustainthe max V_(RESET) that will be seen during the circuit operation.Furthermore, since the clamp/reset circuit 104 is coupled to powerswitch 108, the V_(RESET) voltage is one component of the voltage acrossthe power switch 108 during the power switch off period, which maytherefore also rise with falling V_(IN). The maximum reverse voltageapplied across output diode 111 in FIG. 1, is also proportional to themaximum reset voltage V_(RESET) since this voltage, plus output voltage116 is applied across diode 111 divided by the turns ratio of theforward energy transfer element n_(FWD).

FIG. 2 shows generally a schematic of one embodiment of a circuitbenefiting from the teachings of the present invention. In oneembodiment, a circuit benefiting from the teachings of the presentinvention includes voltage control circuitry coupled to the forwardenergy transfer element in accordance with the teachings of the presentinvention, which is adapted to reduce the voltage across the forwardinput winding, substantially to zero, when the power supply inputvoltage falls below a first threshold value, or is adapted to increasethe voltage across the flyback input winding, substantially to equal thepower converter input voltage when the power switch is on when the powersupply input voltage falls below a first threshold value.

To illustrate, compared to the circuit of FIG. 1, voltage controlcircuitry including for example a switch 223 is coupled across forwardinput winding 206, with Vin sense circuit 224 coupled to the switch 223and the converter input rails 201 and 203. Switch 223 could be asemiconductor switch such as a MOSFET or bipolar transistor in apractical implementation. The Vin sense circuit 224 detects when inputvoltage Vin 202 is below a threshold value and shorts the forward inputwinding 206 by closing switch 223.

In practice, Vin sense circuit 224 may detect a first level of Vin 202to determine when to close switch 223 and may detect a second level ofVin 202 to determine when to open switch 223. This use of first andsecond voltage levels is used for example to introduce hysteresis tomaintain V_(FWD) at substantially zero until the input voltage Vin 202at the power converter input rises above the second level of Vin 202. Inone embodiment, the first and second levels may have the same value, inwhich case the hysteresis would be zero. Shorting input winding 206 isequivalent to reducing the forward input number of turns to zero andtherefore the forward energy transfer element turns ratio to zero inequation (6). Equation (7) then reduces to: $\begin{matrix}{{\frac{V_{OUT}}{V_{IN}}❘_{n_{FWD} = 0}} = \frac{D}{n_{FLY}\left( {1 - D} \right)}} & (9)\end{matrix}$

Equation (9) describes the transfer characteristic of a flybackconverter, since the forward converter of FIG. 2 has effectively beenremoved from the system once switch 223 is closed. This has theadvantage that, from equation (4), since V_(FWD) is now substantiallyzero, V_(RESET) is also substantially zero. As a result, the voltagestress on both the power switch 208 and forward output diode 211 arealso reduced.

A benefit of effectively eliminating the forward energy transfer elementunder conditions of low input voltage can also be seen by rearrangingequation (7) to express the minimum input voltage possible for a givenmaximum power switch duty cycle: $\begin{matrix}{V_{INMIN} = {V_{O}\left\lbrack {n_{FWD} + {n_{FLY}\left( \frac{1 - D_{MAX}}{D_{MAX}} \right)}} \right\rbrack}} & (10)\end{matrix}$From equation (10), the value of V_(INMIN) is reduced when n_(FWD) iszero, extending the operating range of the power converter to lowerinput voltages than would otherwise be the case. It is appreciated thatswitch 223 could also be coupled across a part of forward input winding206 instead of the complete winding 206 as shown in FIG. 2. In this casethe voltage across input winding 206 could be reduced to substantiallyzero when switch 223 is closed, allowing the benefits of the presentinvention to be realized.

FIG. 3 shows generally another embodiment of a circuit benefiting fromthe teachings of the present invention, which also includes voltagecontrol circuitry coupled to the forward energy transfer element inaccordance with the teachings of the present invention. For instance, inthe embodiment of FIG. 3, the output winding 319 of the forward energytransfer element 310 may be shorted using voltage control circuitryincluding for example switch 323. If the input winding 306 and outputwinding 319 of forward energy transfer element 310 are perfectlymagnetically coupled, applying a short using switch 323 is electricallyequivalent to the short applied across input winding 206 in FIG. 2. Ifinput and output windings 306 and 319, respectively, are not perfectlycoupled in one embodiment, the voltage across forward input winding 306will still be substantially zero when switch 323 is closed. This willallow the benefits of the present invention to be realized using a lowervoltage switch 323 across the forward output winding 319 rather than thehigh voltage switch 223 required in the circuit of FIG. 2.

In the embodiment of FIG. 3, the value of the input voltage 302 isdetermined through V sense circuit 324, which is also positioned on theoutput side of the power converter. V sense circuit 324 senses thevoltage across the flyback output winding 320 when the power switch 306is on. The voltage across flyback winding 320 can be related to theinput voltage 302 using equation (1) when power switch 306 is on. Thevoltage across output winding 320, is equal to V_(FLY) 321 divided bythe turns ratio N_(FLY) of the flyback energy transfer element. Sincethe value of V_(FWD) is also known in a particular design, being thevalue of Vout 316 plus the forward voltage drop of diode 311, multipliedby the forward energy transfer element turns ratio n_(FOR), the inputvoltage 302 can be derived from the voltage sensed by circuit 324. Asuitable threshold value of Vin 302 can therefore be selected to turn onswitch 323, providing benefits equivalent to those described withreference to the circuit of FIG. 2 in accordance with the teachings ofthe present invention. In another embodiment of the present invention, asecondary switch such as switch 323 in FIG. 3 may be coupled to aprimary Vin sense circuit such as circuit 224 in FIG. 2.

FIG. 4 shows generally yet another embodiment of a circuit benefitingfrom the teachings of the present invention which also includes voltagecontrol circuitry in accordance with the teachings of the presentinvention. For example, as shown in the embodiment depicted in FIG. 4,the voltage control circuitry includes an output winding 423, wound inthe opposite phase to output winding 419, which has been added to theoutput of the forward energy transfer element 410 and an output diode424, which is coupled to this output winding 423.

In operation, since winding 423 is wound in the opposite phase towinding 419, a voltage 422 Voff is generated across winding 423 duringthe off time of the power switch 408 with a polarity that will forwardbias output diode 424 if voltage 422 is high enough. However, thevoltage 422 is proportional to the VRESET voltage 421 appearing acrossthe input forward winding 406 during the off time of power switch 408.As noted above with reference to the relationship of equations (4) and(8), the reset voltage 421 increases as input voltage 402 reduces.

At some value of input voltage 402, depending on the turns ratio ofwindings 406 and 423, the value of voltage 422 during the off time ofpower switch 408, therefore reaches a value where output diode 424conducts. At this point, the reset voltage 421 across winding 406 offorward energy transfer element 410 is clamped and cannot increase, evenif input voltage 402 decreases further.

The consequence of reset voltage 421 being clamped in this way is thatthe magnetic flux within energy transfer element 410 is not necessarilyreset to its initial value before the start of the next switching periodwhen power switch 408 turns on. If energy transfer element 410 does notreset, over a number of switching cycles, the magnetic flux in themagnetic core of forward energy transfer element 410 builds up until themagnetic material of the forward energy transfer element 410 saturatesor starts to saturate. In the context of this description, starting tosaturate refers to the process whereby the magnetic flux in the core ofthe energy transfer element is not fully reset ot its initial value andso builds up over a number of switching cycles but is not high enough tofully saturate the magnetic core material. Once the forward energytransfer element 410 is saturated or starts to saturate, it can nolonger deliver energy to the output of the power converter.

Under these circumstances, the electrical impedance of forward inputwinding 406 falls to substantially zero and therefore effectivelybecomes a short circuit. Therefore, the voltage across the forward inputwinding is reduced, substantially to zero, as a result of the saturationor as a result of forward energy transfer element 410 starting tosaturate when the power supply input voltage falls below a firstthreshold value in accordance with the teachings of the presentinvention.

In a practical circuit, some voltage does appear across forward inputwinding 406 during the power switch 408 on time due to windingimpedance. In addition, since voltage Voff 422 is finite, there is afinite reset voltage during the power switch 408 off time, which reducesthe magnetic flux somewhat before the beginning of the next switchingcycle. This requires that some additional flux is required at thebeginning of the next switching cycle to saturate the forward energytransfer element 410. However, for practical purposes the voltage acrossforward input winding 406 is reduced enough that the circuit thereforebenefits from the teachings of the present invention as described withreference to FIGS. 2 and 3.

In one embodiment, the exact calculation of the number of turns ofoutput winding 423 and the resulting threshold value of input voltage302 at which the forward energy transfer element 410 begins to saturate,is affected by factors such as the type of output diodes 411 and 424used, which influence the voltage across these components when theyconduct. Other factors include the impedance of energy transfer elementwindings and the magnetic material used in and the physical size of themagnetic core of the forward energy transfer element 410. These factorsare consistent once a design is finalized but may vary betweenindividual circuit designs. As such, the number of turns used in outputwinding 423 in one embodiment is determined by prototype constructionand bench testing in order to satisfy the requirements of the particularapplication being addressed.

Other factors such as for example the core material used in forwardenergy transfer element 410 have the effect that although the magneticflux in the magnetic core of forward energy transfer element 410 is nolonger reset to its initial value when the input voltage 402 falls belowa threshold value, the input voltage 402 will need to fall furtherbefore the forward energy transfer element 410 is fully saturated. Inoperation, the power converter 400 may therefore transition fromoperation as a flyforward converter to operation as effectively aflyback converter over a range of input voltages.

The description of the embodiments discussed above describe benefits ofreducing the voltage across for example forward input windings 206, 306and 406 to substantially zero during the on time of power switch 208,308 and 408. This aspect is equivalent to increasing the voltage acrossflyback input windings 205, 305 and 405 to substantially equal converterinput voltages 202, 302 and 402 assuming that the voltage across powerswitches 208, 308 and 408 is substantially zero when they are on.

FIG. 5 shows generally still another embodiment of a circuit 500benefiting from the teachings of the present invention. As shown in thedepicted embodiment, an integrated circuit 502 includes both a powerswitch and control circuit, which controls the duty cycle of the powerswitch in response to a signal provided by feedback circuit 513. In oneembodiment, integrated circuit 502 may also receive other signals,including but not limited to for example a signal responsive to theinstantaneous current flowing in the power switch, which are notdetailed in this description so as not to obscure the teachings of thepresent invention. In one embodiment, circuit 500 also includes anenergy transfer element 506, which is a forward energy transfer element.Circuit 500 further includes an energy transfer element 504, which is aflyback energy transfer element.

In one embodiment, circuit 500 utilizes a technique similar to thatintroduced above with reference to FIG. 4. In the depicted embodiment,an output winding 514, wound in the opposite phase to forward outputwinding 507, is also included as part of the forward energy transferelement 506, which clamps the maximum reset voltage across the inputwinding 511 when diode 508 conducts into power conversion output rail501. In one embodiment, diode 508 conducts into power conversion outputrail 501 at a threshold value of the input voltage 512 determined by theturns ratio of forward energy transfer element 506.

By limiting the reset voltage across input winding 511, the forwardenergy transfer element 506 saturates allowing circuit 500 to benefitfrom the teachings of the present invention as described above withreference to FIG. 4. In one embodiment, the forward energy transferelement 506 saturates when the relationship in equation (4) can nolonger be maintained.

To illustrate, with the winding turns shown, if output diode 508 isassumed to have a forward voltage drop of for example 0.5V, with a Vout514 of for example 12V and a winding turns ratio of for example 20between forward input winding 511 and output winding 514, the outputdiode 508 will start conducting when the reset voltage across forwardinput winding 511 is (12+0.5)×20=250 Volts. This therefore clamps thevoltage across winding 514, which in turn defines the maximum voltagethat can be developed across forward input winding 511 during the offtime of the power switch within 502.

In one embodiment, the forward energy transfer element 506 has a turnsratio between windings 511 and 507 of for example n_(FWD)=20/2=10. Againassuming for example a forward voltage drop of 0.5V across diode 508,V_(FWD) across forward winding 511 will be (12+0.5)×10=125V. Using theseexample values of V_(FWD)=125V and V_(RESET)=250V and rearrangingequation (4) yields a duty cycle D of 0.67. Using for example Vout=12V,n_(FWD)=10, n_(FLY)=30/3=10 and D=0.67 and rearranging equation (7)yields, V_(IN)=179 Volts which is the input voltage 512 value at whichsaturation of forward energy transfer element 506 is initiated.

FIG. 6 shows generally another embodiment of a circuit benefiting fromthe teachings of the present invention, which includes voltage controlcircuitry coupled to the forward energy transfer element 610 inaccordance with the teachings of the present invention. For instance, inthe embodiment of FIG. 6, the winding 607, which is wound in the samepolarity as forward input winding 606, may be shorted using voltagecontrol circuitry including for example switch 625. If the winding 606and winding 607 of forward energy transfer element 610 are perfectlymagnetically coupled, applying a short using switch 625 is electricallyequivalent to the short applied across input winding 206 in FIG. 2. Ifwindings 606 and 607, are not perfectly coupled in one embodiment, thevoltage across forward input winding 606 will still be substantiallyzero when switch 625 is closed. This will allow the benefits of thepresent invention to be realized using a lower voltage switch 625 acrossthe winding 607 which can be constructed with fewer winding turns thanwinding 606, rather than the high voltage switch 223 required in thecircuit of FIG. 2 which is applied across the forward input winding 206.

In the embodiment of FIG. 6, the value of the input voltage 602 isdetermined through Vin sense circuit 624. Vin sense circuit 624 iscoupled to the switch 625 and the converter input rails 601 and 603.Switch 625 could be a semiconductor switch such as a MOSFET or bipolartransistor in a practical implementation. Depending on the type ofswitch 625 used, a diode 623 may be required to prevent reverseconduction through switch 625 during the off period of power switch 608,however with other types of switch, diode 623 would not be necessary.The Vin sense circuit 624 detects when input voltage Vin 602 is below athreshold value and shorts the winding 67 by closing switch 223.

In practice, Vin sense circuit 624 may detect a first level of Vin 602to determine when to close switch 623 and may detect a second level ofVin 602 to determine when to open switch 625. This use of first andsecond voltage levels is used for example to introduce hysteresis.

FIG. 7 shows generally another embodiment of a circuit benefiting fromthe teachings of the present invention, which includes voltage controlcircuitry coupled to the forward energy transfer element 710 inaccordance with the teachings of the present invention. For instance, inthe embodiment of FIG. 7, a winding 707, which is wound in the oppositepolarity to forward input winding 706, may be shorted using voltagecontrol circuitry including for example switch 725. By closing switch725, the reset voltage appearing across winding 706 during the off timeof power switch 708, is clamped to a low value, preventing the magneticflux within the magnetic core of energy transfer element 710 to reset toits initial value before the beginning of the next switching cycle. Asdescribed above with reference to FIG. 4, under these conditions themagnetic core of energy transfer element 710 saturates, causing theelectrical impedance of forward input winding 706 to fall tosubstantially zero. The embodiment shown in FIG. 7 therefore allows thebenefits of the present invention to be realized using a low voltageswitch 725 as winding 707 can be constructed with a low number ofwinding turns. As will be known to one skilled in the art, the winding707 could also be coupled on the output of energy transfer element 710and the switch 725 could for example be switched using a V sense circuitsimilar the circuit 324 shown in FIG. 3.

In the embodiment of FIG. 7, the value of the input voltage 702 isdetermined through Vin sense circuit 724. Vin sense circuit 724 iscoupled to the switch 725 and the converter input rails 701 and 703.Switch 725 could be a semiconductor switch such as a MOSFET or bipolartransistor in a practical implementation. Depending on the type ofswitch 725 used, a diode 723 may be required to prevent reverseconduction through switch 725 during the on period of power switch 708,however with other types of switch, diode 723 would not be necessary.The Vin sense circuit 724 detects when input voltage Vin 702 is below athreshold value to determine the point at which switch 725 is turned on.

In practice, Vin sense circuit 724 may detect a first level of Vin 702to determine when to close switch 723 and may detect a second level ofVin 702 to determine when to open switch 725. This use of first andsecond voltage levels is used for example to introduce hysteresis.

In the foregoing detailed description, the present invention has beendescribed with reference to specific exemplary embodiments thereof. Itwill, however, be evident that various modifications and changes may bemade thereto without departing from the broader spirit and scope of thepresent invention. The present specification and figures are accordinglyto be regarded as illustrative rather than restrictive.

1. A power converter, comprising: a positive input supply rail and anegative input supply rail, wherein a power converter input voltage isto be applied between the positive and negative input supply rails; aflyback energy transfer element having a flyback input winding; aforward energy transfer element having a forward input winding, whereinthe flyback and forward input windings are coupled between the positiveand negative input supply rails; and voltage control circuitry coupledto the forward energy transfer element to reduce a voltage across theforward input winding, substantially to zero, when the power converterinput voltage falls below a first threshold value.
 2. The powerconverter of claim 1 wherein the voltage control circuitry is adapted tomaintain the voltage across the forward input winding at substantiallyzero until the power converer input voltage rises above a secondthreshold value.
 3. The power converter of claim 2 wherein the secondthreshold value is greater than the first threshold value.
 4. The powerconverter of claim 2 wherein first and second threshold values aresubstantially equal.
 5. The power converter of claim 1 wherein thevoltage control circuitry comprises a switch coupled across the forwardinput winding.
 6. The power converter of claim 1 wherein the voltagecontrol circuitry comprises a switch coupled across an output winding ofthe forward energy transfer element.
 7. The power converter of claim 1wherein the voltage control circuitry is adapted to cause the forwardenergy transfer element to saturate.
 8. The power converter of claim 1wherein the voltage control circuitry is adapted to cause the forwardenergy transfer element to start to saturate.
 9. The power converter ofclaim 1 further comprising a power switch coupled to at least one of theforward input winding or flyback input winding.
 10. The power converterof claim 9 wherein the power switch is further coupled to one of thepositive or negative input rails.
 11. The power converter of claim 9further comprising a control circuit coupled to the power switch;wherein the control circuit and power switch are included in anintegrated circuit.
 12. The power converter of claim 11 the controlcircuit and power switch are monolithically integrated as part of theintegrated circuit.
 13. A power converter, comprising: a positive inputsupply rail and a negative input supply rail, wherein a power converterinput voltage is to be applied between the positive and negative inputsupply rails; a flyback energy transfer element having a flyback inputwinding; a forward energy transfer element having a forward inputwinding; a power switch, wherein the flyback and forward input windingsand the power switch are coupled between the positive and negative inputsupply rails, wherein the voltage across the power switch issubstantially zero when the power switch is on; and voltage controlcircuitry coupled to the forward energy transfer element to increase thevoltage across the flyback input winding, substantially to equal thepower converter input voltage when the power switch is on when the powerconverter input voltage falls below a first threshold value
 14. Thepower converter of claim 13 wherein the voltage control circuitry isadapted to maintain the voltage across the flyback input winding,substantially to equal the power converter input voltage, until thepower converter input voltage rises above a second threshold value. 15.The power converter of claim 14 wherein the second threshold value isgreater than the first threshold value.
 16. The power converter of claim14 wherein the first and second threshold values are substantiallyequal.
 17. The power converter of claim 13 wherein the voltage controlcircuitry comprises a switch coupled across the forward input winding.18. The power converter of claim 13 wherein the voltage controlcircuitry comprises a switch coupled across an output winding of theforward energy transfer element
 19. The power converter of claim 13wherein the voltage control circuitry is adapted to cause the forwardenergy transfer element to saturate.
 20. The power converter of claim 13wherein the voltage control circuitry is adapted to cause the forwardenergy transfer element to start to saturate.
 21. The power converter ofclaim 13 further comprising a control circuit coupled to the powerswitch; wherein the control circuit and power switch form part of anintegrated circuit.
 22. The power converter of claim 21 wherein thecontrol circuit and power switch are monolithically integrated as partof the integrated circuit.
 23. A method, comprising: receiving an inputvoltage at an input of a flyforward converter; switching a power switchof the flyforward converter to regulate an output of the flyforwardconverter; and increasing a voltage across an input winding of a flybackenergy transfer element of the flyforward converter to a valuesubstantially equal to the input voltage at the input of the flyforwardconverter when the power switch is on when the input voltage at theinput of the flyforward converter falls below a threshold value.
 24. Themethod of claim 23 further comprising maintaining a voltage across aninput winding of a flyback energy transfer element of the flyforwardconverter to a value substantially equal to the input voltage at theinput of the flyforward converter until the input voltage at the inputof the flyforward converter rises above a second threshold value.
 25. Amethod, comprising: receiving an input voltage at an input of aflyforward converter; switching a power switch of the flyforwardconverter to regulate an output voltage of the flyforward converter; andreducing a voltage across an input winding of a forward energy transferelement of the flyforward converter to substantially zero when the inputvoltage at the input of the flyforward converter falls below a thresholdvalue.
 26. The method of claim 25 further comprising maintaining thevoltage across the input winding of the forward energy transfer elementof the flyforward converter at substantially zero until the inputvoltage at the input of the flyforward converter rises above a secondthreshold value.
 27. A method, comprising: receiving an input voltage atan input of a flyforward converter; switching a power switch of theflyforward converter to regulate an output of the flyforward converter;and saturating a forward energy transfer element of the flyforwardconverter when the input voltage at the input of the flyforwardconverter falls below a threshold value.
 28. A method, comprising:receiving an input voltage at an input of a flyforward converter;switching a power switch of the flyforward converter to regulate anoutput of the flyforward converter; and starting saturation of a forwardenergy transfer element of the flyforward converter when the inputvoltage at the input of the flyforward converter falls below a thresholdvalue.